Berlinite is anhydrous aluminum phosphate in the quartz form. It may be prepared by heating precipitated, amorphous aluminum phosphate to high temperature (823.degree. K) in the presence of lithium fluoride. J. Papailhau, Compt. Rend., 242, 1191 (1956). Berlinite may also be prepared by direct reaction of alumina with phosphoric acid. M. Tsuhako et al., Nippon Kagku Zasshi, 92, 318 (1971). It is also known to employ aluminum trichloride in the reaction with phosphoric acid and sodium hydroxide (CA74:27559d).
Berlinite is of commercial value due to its piezoelectric and electronic properties. High quality berlinite may be suitably employed as an alternative to quartz in surface acoustic wave piezoelectric devices. For such applications it is imperative that the berlinite crystals be of extremely high purity. In particular, iron, sodium and silicon impurities are desirably substantially absent, and water content must be extremely low. Where relatively large nutrient crystals of berlinite are employed as seed crystals, the avoidance of water entrapment is highly important.
Previously known methods of preparing berlinite have not proven acceptable in order to prepare an extremely high purity product suitable for electronic applications. Accordingly, it would be desirable to provide a process for the preparation of high purity berlinite. It would further be desirable to provide a process that allows for the ready preparation of high purity berlinite at modest cost in both equipment employed and in energy consumed.